Lacquer Checking in Crotch Mahogany Veneer
Periodically, checking develops weeks or months later as moisture content of veneer/core changes. The process of making the raw table top (2-ply veneer on MDF with 2-ply backing) is relatively climate-controlled. The help I am looking for is in the finishing area, not the manufacturing area. I understand there are two ways to reduce the chance of checking: 1) Apply a substance to the raw veneer top which fills the pores and locks the veneer into place, thus not allowing it to move during moisture/temperature changes, and 2) using a flexible sealer and/or top coat lacquer that will flex with the veneer as it expands and/or contracts. What is the best process and/or chemicals/additives that will help me reduce my checking problem?
From contributor E:
I work for a manufacturer of high end conference tables as well. The problem with lacquer on mahogany is that you cannot build it up enough to fill the grain, and when you do, you exceed the mil limit, which is usually 4 mils. It sounds like, if you're putting 5-6 coats on, you've exceeded 4 mils.
Also, are you positive that it's checking? It might also be shrinkage. Lacquer will continue to shrink for a month after you spray it. If you are doing high gloss, this is a big problem because what looks great today won't look great in a month. My suggestion is to try a two-component polyurethane. You can spray that up to 8 mils without worrying about finish failure. The shrinkage with a good 2kpu is very minimal in comparison to nitrocellulose lacquer. Also, the stuff rubs like a champ. You don't really need to worry about getting it hot with the buffing wheel as you would lacquer.
From contributor J:
P.S. They were originally using nitrocellulose lacquer when they were having all these problems, and it was always on the exotic veneers like crotch mahogany and bubinga.
From contributor T:
If you're not ready for the learning curve to 2-part, use automotive clear acrylic lacquer. Did many burled walnut veneered interior trim parts for Jaguars using paste wood filler and acrylic all the way. Many coats, wet-sanded after multiple coats and drying, repeated and left off the gun finish. Buffed a few, no complaints.
From contributor J:
What learning curve? Mix part A with part B 2 to 1. Spray like lacquer, two coats, scuff with 220, 2 coats of lacquer scuff and final top coat. Let dry and polish.
From contributor B:
The veneer itself could be checking and forcing the lacquer to crack as well. The coats of lacquer appear to be excessive and could cause cracking. I would recommend a system more suited for this application, perhaps a conversion varnish, or a 2 part urethane. Something that has flexibility, like a CV or urethane, can sometimes overcome the movement of veneers.
From contributor R:
I know that you are looking for answers about finishing, but I can't help wonder if the problem stems from the manufacturing end of things.
Some questions to consider before you blame the finish for the failure...
To start with, what is the moisture content of the veneer and substrate? Properly prepared veneer should simply not shrink that much. If the veneer and substrate are not within normal moisture levels, you are asking the finish to compensate for inadequately prepared materials.
What kind of glue are you using to apply the veneer to the substrate? If it is a contact cement or an aliphatic resin, this could be the cause of the creep in the veneer. Try a glue that has a rigid glue line like a 2 part product such as Unibond 800 or a Resorcinol.
Finally, do you coat the underside of the table? Moisture could be entering here and causing the assembly to move.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
If you apply crotch mahogany veneer/45 degree movingue veneer on MDF, the veneer moving due to moisture is reduced. It is better if you use the mahogany veneer/ +45 degree movingue/-45 degree movingue. Applying an adhesive to the raw veneer top which fills the pores and locks the veneer into place is not always a good solution. This can cause a dead look finish because you use NGR stain as color base. Also, in some climate changes, it can lead to serious cracks. If applying epoxy glue, I am afraid it is unavailable because some finishes require the non-yellowing look.
After hot press, the laminated veneer panels should not be pre-sanded to remove the excessive cured glue. They should be conditioned at least thirty minutes – one hour depending on thickness of the panels. This is to avoid breaking the glue bone between veneers and MDF, especially for the ureformal dehyde glue system. The hidden defects of UF glue can reduce the stability of mahogany veneer in checking.
The total dry thickness of finish should be in 3 - 5 mils. If over 5 mils, the finish is in high risk of checking, especially finishes on sensitive moisture crotch mahogany veneer substrate.
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